Gas Hydrates 1 E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 521

Veröffentlichungsjahr: 2017

Beschreibung

Gas hydrates, or clathrate hydrates, are crystalline solids resembling ice, in which small (guest) molecules, typically gases, are trapped inside cavities formed by hydrogen-bonded water (host) molecules. They form and remain stable under low temperatures – often well below ambient conditions – and high pressures ranging from a few bar to hundreds of bar, depending on the guest molecule. Their presence is ubiquitous on Earth, in deep-marine sediments and in permafrost regions, as well as in outer space, on planets or comets. In addition to water, they can be synthesized with organic species as host molecules, resulting in milder stability conditions: these are referred to as semi-clathrate hydrates. Clathrate and semi-clathrate hydrates are being considered for applications as diverse as gas storage and separation, cold storage and transport and water treatment.

This book is the first of two edited volumes, with chapters on the experimental and modeling tools used for characterizing and predicting the unique molecular, thermodynamic and kinetic properties of gas hydrates (Volume 1) and on gas hydrates in their natural environment and for potential industrial applications (Volume 2).

Leseprobe

Table of Contents

Cover

Title

Copyright

Preface

1 Neutron Scattering of Clathrate and Semiclathrate Hydrates

1.1. Introduction

1.2. Neutron scattering

1.3. Probing structural and dynamical properties of gas hydrates

1.4. Selected examples

1.5. Concluding remarks

1.6. Bibliography

2 Spectroscopy of Gas Hydrates: From Fundamental Aspects to Chemical Engineering, Geophysical and Astrophysical Applications

2.1. Introduction

2.2. Vibrational spectrum

2.3. Applications to the investigation of formation mechanism

2.4. NGHs: contribution of spectroscopy

2.5. Clathrate hydrates in astrophysical environments

2.6. Concluding remarks

2.7. Bibliography

3 High-Resolution Optical Microscopy of Gas Hydrates

3.1. Introduction

3.2. Optical methods

3.3. Selected examples

3.4. Concluding remarks

3.5. Acknowledgments

3.6. Bibliography

4 Calorimetric Characterization of Clathrate and Semiclathrate Hydrates

4.1. Introduction

4.2. DTA and differential scanning calorimetry

4.3. Phase equilibrium determination in hydrate systems using pressure-controlled TDA and DSC

4.4. Measuring the heat of dissociation and heat capacity of gas hydrates

4.5. Measuring the kinetics of hydrate formation

4.6. Conclusion

4.7. Bibliography

5 Thermodynamic Modeling of Solid-Fluid Equilibria: From Pure Solid Phases to Gas Semiclathrate Hydrates

5.1. Introduction

5.2. Solid-fluid equilibrium between a fluid mixture and a pure solid phase

5.3. Solid-liquid equilibrium between a liquid mixture and a solid solution

5.4. SLE between a liquid mixture and a solid compound

5.5. Thermodynamic model for gas semiclathrate hydrates

5.6. Conclusion

5.7. Bibliography

6 Volume and Non-Equilibrium Crystallization of Clathrate Hydrates

6.1. Introduction

6.2. Driving force and evidence for non-equilibrium gas hydrate crystallization

6.3. Non-equilibrium hydrate formation?

6.4. Modeling gas to hydrate transfer: equilibrium thermodynamics versus kinetics

6.5. Non-equilibrium flash calculations

6.6. A kinetic Langmuir based modeling approach

6.7. Conclusion

6.8. Nomenclature

6.9. Bibliography

List of Authors

Index

End User License Agreement

List of Tables

1 Neutron Scattering of Clathrate and Semiclathrate Hydrates

Table 1.1. Examples of scattering lengths (b in fm) and cross-section (σ in barns) for some selected chemical species [SEA 92]

2 Spectroscopy of Gas Hydrates: From Fundamental Aspects to Chemical Engineering, Geophysical and Astrophysical Applications

Table 2.1. Typical abundances of molecules in cometary and interstellar ices (relative to water). Adapted from Table 3 in [MUM 11]

3 High-Resolution Optical Microscopy of Gas Hydrates

Table 3.1. Temperature interval (above Teq) where “superheated” hydrates are observed to be stable

4 Calorimetric Characterization of Clathrate and Semiclathrate Hydrates

Table 4.1. Systems studied using DTA/DSC for phase diagram determinations

6 Volume and Non-Equilibrium Crystallization of Clathrate Hydrates

Table 6.1. Simulations compared to the experimental results of Le Quang et al. [LEQ 16] for non-stoichiometric and stoichiometric hydrate flash algorithms [BOU 16] (* slow crystallization rate)

Guide

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Series EditorAllain Dollet

Gas Hydrates 1

Fundamentals, Characterization and Modeling

Edited by

Daniel Broseta

Livio Ruffine

Arnaud Desmedt

First published 2017 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUKwww.iste.co.uk

John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USAwww.wiley.com

© ISTE Ltd 2017The rights of Daniel Broseta, Livio Ruffine and Arnaud Desmedt to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2017936797

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-969-4

Cover image: DIC image of cyclopentane hydrate formed on a water drop (with diameter in the mm range) on hydrophilic glass, showing a faceted crust over the water, surrounded by a fine-grained halo, on the substrate under the guest phase (see Chapter 3). Pixel coloring by intensity (dark to light shades) highlights the delicate beauty of hydrate crystals revealed by high resolution microscopy.

Preface

Clathrate hydrates are crystalline inclusion compounds resulting from the hydrogen bonding of water (host) molecules enclosing relatively small (guest) molecules, such as hydrogen, noble gases, carbon dioxide, hydrogen sulfide, methane and other low-molecular-weight hydrocarbons. They form and remain stable at low temperatures – often well below ambient temperature – and high pressures – ranging from a few bar to hundreds of bar, depending on the guest molecule. Long considered either an academic curiosity or a nuisance for oil and gas producers confronted with pipeline blockage, they are now being investigated for applications as diverse as hydrogen or methane storage, gas separation, cold storage and transport, water treatment, etc. The ubiquitous presence of natural gas hydrates not only in the permafrost, but also in deep marine sediments, has been identified, and their role in past and present environmental changes and other geohazards, as well as their potential as an energy source, are under intense scrutiny.

These perspectives are motivating an ever-increasing research effort in the area of gas hydrates, which addresses both fundamental issues and applications. Gas hydrates exhibit fascinating yet poorly understood phenomena. Perhaps the most fascinating feature exhibited by gas hydrates is self-preservation, or the existence of long-lived metastable states in some conditions far from stable thermodynamic equilibrium. Strong departures from equilibrium are also noted in gas hydrate compositions, depending on their formation and kinetic pathways. A proper understanding of these two effects could serve in developing gas storage and selective molecular-capture processes. The memory effect, or the ability of gas hydrates to reform rapidly in an aqueous solution where gas hydrates have been freshly melted, is another puzzling phenomenon. Gas hydrates are likely to be soon exploited for storing gas (guest) molecules or for separating or capturing some of them selectively; yet, the occupancy rates of the different hydrate crystal cavities by the various guest molecules are not fully understood. Very little is known as well on hydrate formation and their stability in the extreme conditions (e.g. low or high pressures) such as on extraterrestrial bodies like comets and planets. How hydrates interact with substrates is a topic of prime interest for understanding not only the behavior of hydrates in sediments, but also why some mesoporous particles act as hydrate promoters. Nucleation and growth processes are still unsettled issues, together with the mechanisms by which additives (co-guest molecules, surfactants, polymers, particles, etc.) promote or inhibit hydrate formation. Depending on the application, these additives are needed to either accelerate or slow down the crystallization process; but their selection is still carried out on a very empirical basis. This book gathers contributions from scientists who actively work in complementary areas of gas hydrate research. They have been meeting and exchanging views regularly over the past few years at a national (French) level, and recently at a European level, within the COST Action MIGRATE (Marine gas hydrate – an indigenous resource of natural gas for Europe). This book is somehow the written expression of those meetings and exchanges. It is divided into two volumes: the first (and present) volume is devoted to the fundamentals, characterization and modeling of gas hydrates, whereas the second volume will focus on gas hydrates in their natural environment and for industrial applications.

The present volume starts with an extensive presentation of the experimental tools capable of probing small spatial and temporal scales: neutron scattering (Chapter 1), spectroscopy (Chapter 2) and optical microscopy (Chapter 3). In addition to providing fundamental insights into structural and dynamical properties, these tools have allowed considerable progress in the understanding of the molecular and mesoscopic mechanisms governing hydrate formation and growth. Moving to larger scales, the calorimetric tools used to measure heat and related thermodynamic properties are described in Chapter 4. Chapter 5 provides a comprehensive view of the thermodynamic modelling of solid-fluid equilibria, from pure solid phases to gas semiclathrate hydrates. Finally, Chapter 6 presents a novel approach coupling thermodynamics and kinetics to describe the non-equilibrium effects occurring during hydrate formation, with a focus on the evolution of the composition of the hydrate phase. Most of these chapters extend their scope to semi-clathrates, in which gas or small molecules still occupy the crystal cavities, but the cavities themselves consist of water and organic species, such as quaternary ammonium salts, strong acids or bases. These semiclathrates hold great promise from a practical point of view, because the temperature and pressure conditions of their formation and stability are closer to the ambient than their hydrate counterparts.

Volume 2 addresses geoscience issues and potential industrial applications. It deals with marine gas hydrates through a multidisciplinary lens, integrating both field studies and laboratory work and analyses, with a focus on the instrumentations and methods used to investigate the dynamics of natural deposits. This is followed by the description of the geochemical models used for investigating the temporal and spatial behavior of hydrate deposits. Finally, potential industrial applications of clathrate and semiclathrate hydrates are also presented in that volume.

To conclude, we would like to warmly thank all the contributors to the present volume for taking the time to write concise and clear introductions to their fields.

Daniel BROSETALivio RUFFINEArnaud DESMEDTApril 2017

1Neutron Scattering of Clathrate and Semiclathrate Hydrates

1.1. Introduction

Neutron scattering is a standard tool when dealing with the microscopic properties of the condensed matter at the atomic level. This comes from the fact that the neutron matches with the distances and energy scales, and thus with the microscopic properties of most solids and liquids. Neutrons, with wavelengths in the order of angstroms, are capable of probing molecular structures and motions and increasingly find applications in a wide array of scientific fields, including biochemistry, biology, biotechnology, cultural heritage materials, earth and environmental sciences, engineering, material sciences, mineralogy, molecular chemistry, solid state and soft matter physics.

The striking features of neutrons can be summarized as follows. Neutrons are neutral particles. They interact with other nuclei rather than with electronic clouds. They have (de Broglie) wavelengths in the range of interatomic distances. They have an intrinsic magnetic moment (a spin) that interacts with the unpaired electrons of magnetic atoms. Their mass is in the atomic mass range. They carry, thus, similar energies and momentum than those of condensed matter, and more specifically of gas hydrates.

As gas hydrates are mainly constituted of light elements (H, O, C, etc.), in situ neutron scattering appears as a technique particularly suited to their study. In the case of diffraction (i.e. structural properties), while the identification of these light atoms by X-ray diffraction requires the presence of heavy atoms and is therefore extremely complicated, neutron diffraction (NP) is highly sensitive to them due to the interaction of the neutrons with nuclei rather than with electron clouds. Moreover, most of the matter is “transparent” to neutron beams. Such a feature provides advantages for studying gas hydrates when a heavy sample environment is required (e.g. high pressure, low temperature). For instance, X-ray powder diffraction studies are usually restricted to small sample volumes, as large sample volumes would be associated with a strong absorption and unwanted scattering from the pressure cell. Neutron techniques allow studies of bulk processes in situ in representative volumes, hence with high statistical precision and accuracy [STA 03, HEN 00, GEN 04, FAL 11]. Furthermore, although alteration of some types of ionic clathrate hydrates (or semiclathrates), such as the splitting of the tetra-alkylammonium cations into alkyl radicals [BED 91, BED 96], by X-ray irradiation has been reported, neutrons do not damage sample.

Finally, future developments in gas hydrate science will be based on the understanding, at a fundamental level, of the factors governing the specific properties of gas hydrates. In this respect, the investigation of gas hydrate dynamics is a prerequisite. At a fundamental level, host–guest interactions and coupling effects, as well as anharmonicity, play an important role. These phenomena take place over a broad timescale, typically ranging from femtoseconds to microseconds. Investigating the dynamics (intramolecular vibrations, Brownian dynamics, etc.) of gas hydrates thus requires various complementary techniques, such as NMR or Raman spectroscopy, and indeed inelastic and quasi-elastic neutron scattering (QENS), especially when it comes to encapsulating light elements such as hydrogen or methane in water-rich structures.

In this chapter, the recent contributions of neutron scattering techniques in gas hydrate research are reviewed. After an introduction to neutron scattering techniques and theory, an overview of the accessible information (structural and dynamical properties) by means of neutron scattering is provided. Then, selected examples are presented, which illustrate the invaluable information provided by neutron scattering. Some of these examples are directly related to existing or possible applications of gas hydrates.

1.2. Neutron scattering

Both nuclear and magnetic neutron interactions are weak: strong but at very short length scale for the nuclear interaction and at larger scale for the magnetic interaction. In that respect, the probed sample can be considered as transparent to the neutron beam. This highly non-destructive character combined with the large penetration depth, both allowed because of the weak scattering, is one of the main advantages of this probe.

Nuclear scattering deals with nuclear scale interaction and hence presents no wave vector dependent form factor attenuation allowing to offer high momentum transfers for diffraction or specific techniques such as deep inelastic neutron scattering (also known as neutron Compton scattering).

Neutron spectroscopic techniques range from the diffraction of large objects using small-angle scattering, usually made with long incident wavelengths (cold neutrons), to direct imaging through contrast variation (neutron tomography), usually made with short wavelengths (hot neutrons) and going through ordinary diffraction and inelastic scattering in the intermediate wavelength range.

In that respect, neutron scattering complements without necessarily overlapping the other available spectroscopic techniques such as nuclear magnetic resonance (NMR). If one naturally thinks about X-ray for structure determination, neutrons are very competitive for inelastic scattering and even essential for magnetic scattering both in the diffraction and inelastic modes.

The main drawback that contrasts with the numerous advantages comes from the intrinsic relative flux limitation of neutron sources, and thus, this type of spectroscopy can only be performed at dedicated large-scale facilities.

1.2.1. A basic ideal scattering experiment

In a generic experiment (Figure 1.1), a beam of monochromated neutrons with single energy (Ei) is directed on a sample. The scattered neutrons are collected along direction (angles θ and ϕ) and analyzed by energy difference with the incident energy by using a detector, covering a solid angle ΔΩ of the sphere, which measures the analyzed neutron intensity. The measured intensity in the solid angle spanned by the detector and in a final energy interval ΔEf in this simple gedanken experiment reads:

[1.1]

where Φ stands for the incident flux at the incident energy and η is the efficiency of the detector. The quantity between the identified terms is the double differential scattering cross-section, a surface per unit of energy, which characterizes the interaction of the neutron with the sample or the surface that the sample opposes to the incident beam. Since the intensity has the dimension of count/s, the double differential scattering cross-section can be seen as the ratio of the scattered flux in the given detector per unit energy over the incident flux.

Figure 1.1.Sketch of an ideal scattering experiment. An incident neutron beam of monochromatic energy Ei and wave vector ki is scattered with energy Ef and wave vector kf. For a color version of this figure, see www.iste.co.uk/broseta/hydrates1.zip

1.2.2. Neutron scattering theory

The mathematical development of the neutron scattering technique comes from the more general scattering theory. The interaction of the neutron with a single nucleus is first examined and then the generalization of the theory for an assembly of scatterers is developed. From scattering theory to its application to neutron scattering, the aim is to convince that the scattering of neutrons by the nuclei or by the spins of an ensemble of atoms provides information on the structure and motions of the atoms, i.e. information on the sample under investigation at the atomic level.